Questions about section "Index Signatures"

[JerryFan626]

This chapter talks about:
Using an intersection type to slove combine properties into index signature,but this is not useful.
I have a question, Why not use Union Types(|) to combine this properties?
And, Why TS is not infer "never" type, when you use intersection type(&) to merge two types that cannot possibly intersect.

type FieldState = {
  value: string
}

type FormState =
  { isValid: boolean }
  & { [fieldName: string]: FieldState }